1. Field of the Invention
The present invention relates to piezoelectric resonators using a radial flexural mode vibration.
2. Description of the Related Art
General piezoelectric resonators for use in kilohertz bands, particularly in 100 kHz to 1 MHz use an area expansion mode vibration. In a piezoelectric resonator using area expansion mode vibration, the product of the length Ls of a side and resonant frequency fr is substantially constant, and is represented by the following expression:
Lsxc3x97fr=As
where As represents a constant (frequency constant) and As≈2100 mmkHz. For example, in a resonator having a resonant frequency fr=455 kHz as in an AM filter, the length of a side is represented by Ls=4.62 mm.
In recent years, electronic devices have been in the process of being reduced in size, and it is also necessary that electronic components have small sizes and reduced thicknesses. Under such circumstances, it is not preferable to use an electronic component as described above, in which a side length is about 5 mm.
In addition, the attenuation characteristics of a ladder filter are determined by the ratio between the capacitance of a parallel resonator and the capacitance of a series resonator. In other words, in order to achieve a large attenuation, the inter-terminal capacitance of the parallel resonator is increased, while the inter-terminal capacitance of the series resonator is reduced. However, to increase the inter-terminal capacitance of the parallel resonator, the thickness of the piezoelectric substrate of the parallel resonator must be reduced, so that the mechanical strength decreases. Accordingly, there is a limitation on the range of selectable thicknesses.
In view of the above-described circumstances, the assignee of the present application has proposed a piezoelectric resonator, which is described in Japanese Patent Application No. 11-294491. In this piezoelectric resonator, groups of at least four electrode layers and groups of at least three piezoelectric layers are alternately stacked and in each piezoelectric group, at least two piezoelectric layers are polarized, and in which the electrode layers are connected to one another so that a portion of the piezoelectric layers has an electric field generated in a direction that is identical to the polarization direction of the two piezoelectric layers and another portion of the piezoelectric layers has an electric field generated in a direction that is opposite to the polarization direction of the two piezoelectric layers.
In the entirety of the above-described piezoelectric resonator, radial flexural mode vibration occurs because piezoelectric layers in which the polarization direction and the electric field direction are identical contract, and piezoelectric layers in which the polarization direction and the electric field direction are opposite expand. In this type of piezoelectric resonator using radial flexural mode vibration, dimensions can be reduced at the same resonant frequency, compared with a piezoelectric resonator using area expansion mode vibration. Since the piezoelectric resonator using radial flexural mode vibration has at least four electrode layers, an inter-terminal capacitance can be generated between adjacent electrode layers and can be increased. In addition, the stacking of the piezoelectric layers is performed. Thus, it is ensured that the entirety of the piezoelectric layers has sufficient mechanical strength, even if each piezoelectric layer has a small thickness.
Nevertheless, the piezoelectric resonator using radial flexural mode vibration has the following problems. The piezoelectric layers of this piezoelectric resonator have identical thicknesses. Thus, assuming that the entirety of the piezoelectric resonator as a device is constant, when n piezoelectric layers are stacked, the thickness of one piezoelectric layer is represented by 1/n. At this time, the inter-terminal capacitance is represented by the square of n. Accordingly, a rate of capacitive change is large, and the degree of freedom in design of capacitance is limited.
In addition, to obtain a predetermined capacitance, the thickness of the entire device, the number of layers, material properties, etc., must be changed, and as a result complications in processing steps and increases in costs are inevitable.
In order to overcome the problems described above, preferred embodiments of the present invention provide a very small piezoelectric resonator in which a capacitance is accurately controlled without changing the number of stacked piezoelectric layers and the thickness of the entire resonator.
According to a preferred embodiment of the present invention, a piezoelectric resonator uses radial flexural mode vibration and includes an even number of at least four electrode layers and an odd number of at least three piezoelectric layers which are alternately stacked, wherein at least two piezoelectric layers among the three piezoelectric layers are polarized in a thickness direction, the four electrode layers are connected to one another so that, in at least one piezoelectric layer among the three piezoelectric layers, an electric field is generated in a direction that is identical to the polarization direction of the at least one piezoelectric layer, and in at least another piezoelectric layer among the three piezoelectric layers, an electric field is generated in a direction that is opposite to the polarization direction of the at least another piezoelectric layer, and among the three piezoelectric layers, at least one piezoelectric layer has a thickness that is different from each of the thicknesses of the other piezoelectric layers.
According to preferred embodiments of the present invention, the entirety of a piezoelectric resonator generates radial flexural mode vibration because piezoelectric layers in which polarization directions and electric field directions are identical contract two-dimensionally, and piezoelectric layers in which polarization directions and electric field directions are opposite expand. This piezoelectric resonator using radial flexural mode vibration can be made much smaller than a piezoelectric resonator using area expansion mode vibration, if the same resonant frequency is used. For example, when a resonator has a resonant frequency fr=455 kHz, each side of a resonator using area expansion mode vibration is approximately 4.62 mm long, while each side of a resonator using radial flexural mode vibration can be set at approximately 1.4 mm.
Among the three piezoelectric layers, at least one piezoelectric layer has a different thickness than the other piezoelectric layers. For example, in the case of a three-layer piezoelectric resonator, when the length of a side is represented by Lb, the dielectric constant of the piezoelectric layers is represented by ∈, the thickness of the central piezoelectric layer is represented by t2, and the thicknesses of outer piezoelectric layers are represented by t1 and t3, inter-terminal capacitance Cb is given by the following expression:
Cb=(∈xc2x7∈0xc2x7Lb2) (1/t1+1/t2+1/t3)
where ∈0 represents the permeability of vacuum, and t1+t2+t3=T0.
When t1=t2=t3,
Cb=(∈xc2x7∈0xc2x7Lb2) (9/T0)
If t1=t3=t2/2,
Cb=(∈xc2x7∈0xc2x7Lb2) (10/T0)
Also, when t1=t3=2xc2x7t2,
Cb=(∈xc2x7∈0xc2x7Lb2) (10/T0)
As described above, by setting the thickness of at least one piezoelectric layer to differ from each of the thicknesses of the other piezoelectric layers, inter-terminal capacitance can be changed, without increasing or reducing the number of piezoelectric layers.
Preferably, the three piezoelectric layers have a relationship expressed by:
t1=t3xe2x89xa0t2
By using the above relationship, radial flexural mode vibration is symmetric in top and bottom surface directions because the outer piezoelectric layers have substantially equal thicknesses, so that preferable resonant characteristics can be obtained.
The three piezoelectric layers may have a relationship expressed by:
1 less than t1/t2 less than 3 and 1 less than t3/t2 less than 3
In other words, by setting the thickness of each piezoelectric layer to be larger than the thickness of the central piezoelectric layer, a piezoelectric resonator having large xcex94F (=faxe2x88x92fr) can be obtained and a piezoelectric resonator having a broad bandwidth can be obtained.
Among the three piezoelectric layers, the central piezoelectric layer may be piezoelectrically polarized in the thickness direction.
Alternatively, among the three piezoelectric layers, the central piezoelectric layer may not have piezoelectric characteristics in the thickness direction.
According to preferred embodiments of the present invention, in the range t1/t2 less than 3, wherein t1=t3, large xcex94F can be obtained by setting the central piezoelectric layer to have piezoelectric characteristics in the thickness direction, while in the range t1/t2 greater than 3, large xcex94F can be obtained by setting the central piezoelectric layer not to have piezoelectric characteristics.
When the central piezoelectric layer does not have piezoelectric characteristics, the dimensions of the entire device can be greatly reduced, compared with the case in which the central piezoelectric layer has piezoelectric characteristics.
According to preferred embodiments of the present invention, in a piezoelectric resonator which uses radial flexural mode vibration and which includes an even number of at least four electrode layers and an odd number of at least three piezoelectric layers which are alternately stacked, and in which at least two piezoelectric layers among the three piezoelectric layers are polarized in a thickness direction, at least one piezoelectric layer has a thickness that is different from each of the thicknesses of the other piezoelectric layers. Thus, inter-terminal capacitance can be freely changed, without changing the overall thickness of the device, the number of layers stacked, material properties, and other characteristics.
The capacitance can be sequentially changed based on the thickness of the piezoelectric layers. Thus, the capacitance can be easily controlled to come close to a target capacitance. Accordingly, when mounting the piezoelectric resonator on a circuit board or other mounting surface, impedance matching to an external circuit can be easily performed.
In addition, by changing the thickness of the piezoelectric layers, not only the capacitance but also xcex94f can be adjusted and the bandwidth of the piezoelectric resonator can be adjusted. Also, by changing the thickness of each piezoelectric layer, the device dimension can be changed, even in the case of using the same resonant frequency, the same number of layers stacked, and the same overall thickness.